The concept of radar was discovered late 19th century and was further developed during the first and second world wars where pulsed radars where used in order to detect e.g. hostile aircrafts. A pulsed radar operates at two operation modes, a transmission mode where pulsed, high power radio frequency signals, referred to as transmission signals, are transmitted and a reception mode where the reflected echo of the transmitted transmission signals, which are referred to as reflection signals, reflected signals or reply signals, are received. The carrier frequency of such transmission signal is herein referred to as centre frequency. The transmission signals are transmitted by means of transmission means and the reflected signals are received by means of receiving means.
When a transmission signal is transmitted the transmission signal will give rise to a number of reflected signals originating from all structures, such as e.g. vehicles, humans, and houses, against which the transmitted signal may echo. By means of commonly known methods for filtering out relevant targets from not relevant targets, such as e.g. background scatter and like, the reflected signals resulting from the relevant targets can be analysed. Such commonly known methods may e.g. be based on recognition of certain structures, detecting abnormalities in a known environment or applying various signal filters, but also other methods exists. To give yet one example, prior art comprises various methods for filtering out relevant targets from not relevant targets based on e.g. target characteristics. Such methods will not be addressed more herein but are considered to be part of common general knowledge. When hereinafter referring to targets only targets which are relevant to detect and to analyse are considered. Structures giving rise to background scatter and like are not considered to be targets relevant for detection and further analysis. Targets may e.g. be hostile aircrafts, hostile vehicles or incoming missiles. Generally a number of transmission signals are sent at respective target.
Since only a small portion of a transmitted high power radio frequency signal is reflected when a target is hit the transmission signal has significantly higher power than the reflected reply signal. Thus, for a pulsed radar system designed to detect small targets far away, the receiving means configured to detect the reflected signals have to be able to detect significantly lower powers than what is transmitted by the transmission means. This implies that the receiving means needs to be disconnected in transmission mode since the high powers transmitted by the transmission means otherwise would damage the receiving means. This in turn implies that incident, reflected signals, due to reflected echoes from close range targets, received when the pulsed radar system is set in transmission mode, thus during the time the receiving means are disconnected, are not detected. During the time the receiving means are disconnected the transmission signal travels a certain distance and consequently a close range target located within half the distance the transmission signal travels during this time will not be detected. This is referred to as the minimal measuring range or the blind range of the pulsed radar system, and states the minimum distance from the pulsed radar system a target must be situated at in order to be detected by the pulsed radar system. A typical value of 1 μs for the pulse width of short range radar corresponds to a minimum range of about 150 m, which generally is acceptable.
From the radar equation can be derived that the maximum range of a pulsed radar system is proportional to e.g. the duration of the transmission signal and the maximum output power of the radar, hence the average signal output power of the pulsed radar system during operation. Thus, for the nowadays commonly used pulsed radar systems with solid state transmission the lower maximum output power may be compensated for by prolonging the time period in transmission mode. The longer periods in transmission mode applied for such pulsed radar systems mean more significant blind range and thereby a more significant range within which it is possible to miss a possible target.
Thus, there is a need for further improvements.